The present invention relates to a method of processing wire rod emerging from a hot rolling mill.
The quality of the wire rod depends on various conditions of the wire rod, among which the following, for example, should be mentioned, i.e. dimensions and shape of the cross section of the wire rod, cleanliness of the steel used and particularly absence of inclusions, mechanical properties of the wire rod, particularly its suitability for wiredrawing, in view of the microstructure of the steel used, the transverse and longitudinal homogeneity of the microstructure, and finally the appearance of the rod surface and particularly its possible state of oxidation.
Among the conditions governing the end quality of wire rod, the nature of the scale, the microstructure of the rod, and the homogeneity of the microstructure depend, for a given steel composition, almost exclusively on the treatment undergone by the rod in the last stand of the rolling mill and the course of cooling at the outlet of the rolling mill. Performing such cooling is precisely the problem for which numerous solutions have been suggested.
Rolled rod for drawing generally undergoes a patenting treatment to impart to the wire rod a structure suitable for the drawing operation. Patenting is often carried out at the wire drawing mill. Such treatment is expensive, particularly when applied to small amounts of wire rod and when dealing with wire rod which for certain uses has to undergo two patenting treatments, one at the supply diameter and the other before reaching the final sizes.
To eliminate the patenting operations, which besides being expensive are also inconvenient, numerous procedures for manufacturing steel wire have been suggested. Sometimes the first patenting treatment (pre-patenting) which is applied to the wire rod obtained by hot rolling, can be eliminated by providing suitable cooling at the outlet of the rolling mill. The procedures suggested for performing such cooling are of various types, some of which use special techniques (e.g. fluidizied bed). Most of them, however, comprise a sudden cooling stage followed by a less steep temperature decrease stage sometimes approximating to isothermal treatment.
With the best known suggested procedures, most of which are applied on industrial scale, close or tight coils are no longer formed. The wire rod is instead arranged either flat or vertically in expanded form, i.e. as loose turns. The successive turns of rod are for example displaced on a conveyor on which a liquid coolant is directed or through which a gaseous coolant is blown. The most interesting procedures of this type known at present are generally carried out with wire rod of hard steel (i.e. steel whose carbon content is 0.4 to 0.85%) and allow only the above-mentioned pre-patenting to be eliminated. These procedures reduce the thickness of the layer of scale formed on the wire rod, and they ensure acceptable longitudinal homogeneity of the wire, although it is still inferior to that achievable by patenting, particularly lead patenting.
Generally speaking, controlled cooling procedures are carried out by following a course derived from CCT (continuous cooling-transformation) or TTT (temperature-time-transformation) diagrams in which well-known curves of beginning and end of the transformation of austenite to ferrite-pearlite are plotted against time and temperature. This course is usually chosen in such a way that the transformation commences at a temperature slightly higher than that corresponding to the nose of the curve representing the start of the transformation of the austenite to ferrite-pearlite and is achieved at a temperature substantially equal to that corresponding to the nose of the curve representing the end of the transformation. In this manner, pre-eutectoid ferrite as well as bainite and martensite are prevented at the utmost from being formed.
However, the controlled or regulated cooling procedures known at present are inconvenient because of the practical difficulty of achieving with sufficient certainty the chosen starting point and ending point of the transformation on the CCT or TTT diagram. This difficulty is due to several facts and particularly to the fact that, since the transformation of steel is exothermic, the temperature of the wire rod rises during the transformation, this phenomenon being known as recalescence.